GRAVE WORSHIP WAS A PRACTICE OF NON BELIEVER'S

Aisha(ra) reported: The Messenger of Allaah (pbuh) said during his illness from which he never recovered: Allaah cursed the Jews and the Christians that they took the graves of their prophets as mosques. She (Aisha) reported: Had it not been so, his(Prophet's) grave would have been in an open place, but it could not be due to the fear that it may not be taken as a mosque.

Sahih Bhukari Volume 2, Book 23, Number 414

Sahih Bhukari Volume 2, Book 23, Number 472
Sahih Muslim Haidth #1079
Sahih Muslim Hadith #1082